Hydrogen sulfide is often present in gas streams as a contaminant which prevents the use of the gas for domestic, commercial or industrial purposes. This problem is particularly severe in sour natural gas, which is often produced with H.sub.2 S concentrations from one percent to as high as ninety percent. Over the years, many desulfurization processes have been developed in attempts to produce gas streams substantially free of hydrogen sulfide.
The commercial process most used in the recovery of hydrogen sulfide from an acid gas or sour gas stream and the production of elemental sulfur is the Claus process. The gas stream containing the acid gas is usually treated by solvent extraction or washing out the acid gases with any number of suitable solvents. The extraction or washing step produces a clean, treated gas stream and an acid gas stream. In the Claus process, the acid gas stream, mainly H.sub.2 S, and a controlled stoichiometric quantity of air are fed into a reaction furnace, where one-third of the H.sub.2 S is burned to SO.sub.2. The H.sub.2 S and SO.sub.2 react to form elemental sulfur thermally in the furnace. Also, elemental sulfur is catalytically formed in the reactors which follow the sulfur furnace according to the Clause reaction. One such commercial process is disclosed in Hydrocarbon Processing, April 1982, p. 109.
Another commercial process for the removal of hydrogen sulfide and the partial removal of organic sulfur compounds from natural and industrial gases is the Stretford process. The sour natural or industrial gas is counter-currently washed with an aqueous solution containing sodium carbonate, sodium vanadate and anthraquinone disulfonic acid (ADA). The hydrogen sulfide dissolves in the alkaline solution and is removed to any desired level. The hydrosulfide formed reacts with the 5-valent state vanadium and is oxidized to elemental sulfur. The aqueous solution for extracting the sour gases is regenerated by air blowing, and the reduced vanadium is restored to the 5-valent state through a mechanism involving oxygen transfer via the anthraquinone disulfonic acid. A specific example of this process is set forth in Hydrocarbon Processing, April 1982, p. 112.
Still another process for the conversion of H.sub.2 S to elemental sulfur is the LO-CAT process. This process utilizes a dilute solution of iron held in solution by organic chelating agents. The aqueous solution containing the chelated iron serves as both a catalyst in the overall reaction of H.sub.2 S with oxygen and takes part in the reactions by transfer of electrons. A more specific description of the process is set forth in Hydrocarbon Processing, April 1985, pp. 70 and 71.
U.S. Pat. No. 4,487,753 discloses a process for producing liquid elemental sulfur from a CO.sub.2 -rich gaseous stream containing H.sub.2 S. The gas is contacted with at least a stoichiometric amount of gaseous oxygen in the presence of liquid water with a fixed bed comprising a catalyst selected from the group consisting of a transition metal phthalocyanine compound dispersed on a support at a specified pH and temperature. The patent discloses a preferred support as activated carbon.
U.S. Pat. No. 4,579,727, issued on the application which included the present inventor, discloses a process for recovering elemental sulfur from a hydrogen sulfide containing gas stream by reacting the hydrogen sulfide in the gas stream with a buffered aqueous solution enriched in thiosulfate ions at an initial pH between about 4.5 and 6.5 for a residence time sufficient to react a portion of the hydrogen sulfide to elemental sulfur. The elemental sulfur is them removed and the solution now lean in thiosulfate ions is regenerated by the oxidation of the remaining hydrogen sulfide in the gas stream to deplete the hydrogen sulfide from the gas stream and to regenerate the liquid solution for recycling to the reduction zone.
In many respects, the method of U.S. Pat. No. 4,579,727 has advantages for the removal of H.sub.2 S from gas streams and production of sulfur therefrom. However, this method is carried out in the presence of the entire volume of the gas being treated, requiring the use of large reaction vessels. This method requires the continuous addition of caustic because the process produces a substantial proportion of low value sulfates and the loss of the production of sulfur. Furthermore, with this process, the sulfur product may be contaminated with H.sub.2 S. Finally, if a nitrogen contamination of the gas stream is not allowed, it is necessary to use pure oxygen in the oxidation reaction.